Thermal printing systems are well known and widely used in applications such as facsimile printers, ticket printers and similar devices. Like most printing technologies, thermal printing is irreversible; the printed image is permanent, and neither the substrate nor the ink can be reused.
A number of compounds have been identified for producing inks that may be reversibly colored upon application of heat energy or cooling energy, and which are "bistable": once placed into one or the other state (e.g., transparent or colored), the substance exhibits hysteresis, tending to persist in that state under ordinary ambient conditions. These compounds broadly include low molecular-weight systems, leuco dye systems, lactone-ring systems and urea systems, and fall into two broad categories: materials that are both writable (colored) and erasable (transparent) through heating to different temperatures or with different heat cycles; and materials that require heating and cooling to write and erase. These categories are not always mutually exclusive. Because the compounds tend to exhibit temperature hysteresis and can be formulated to undergo state change at varying transition temperatures, the same chemical system may behave in either fashion depending on the manner of preparation.
For example, U.S. Pat. No. 5,274,460 (to Yamada et al.), the entire disclosure of which is hereby incorporated by reference, describes systems that include a leuco dye, a developing/tone-reducing agent adapted to thermally react with the dye to effect development or tone reduction, and a suitable binder. Exposure of the system to a first thermal energy level (e.g., heating to a high temperature of 200-350.degree. C. for a short duration of 1-3 msec) produces a color, while exposure to a second thermal energy level (e.g., heating to a low temperature of 80-150.degree. C. for a longer duration of 5 msec to 2 sec) renders the system transparent. The developing/tone-reducing agent is a substance having, in the same molecule, a group that exhibits a thermally triggered color-developing property with respect to the leuco dye, and a group that exhibits a thermally triggered tone-reducing property with respect to the leuco dye. The agent may be a salt of a phenolic carboxylic acid and an organic amine, which can be thermally induced to behave as an acid or a base. Under acidic conditions, the lactone rings of the leuco dye open and the molecule becomes colored; exposure to basic conditions closes the rings and restores the original colorless state. See also U.S. Pat. No. 5,178,669.
Other examples of materials that facilitate reversible, bistable recording and erasure through heating to different temperatures include systems using dimerized or trimerized urea developers, as disclosed in U.S. Pat. No. 5,470,816 (to Satake et al.); and the systems disclosed in U.S. Pat. No. 5,432,534 (to Maruyama et al.), which include a thermally sensitive coloring agent such as a triphenylmethane phthalide compound, a fluoran compound, a phenothiazine compound, a leuco auramine compound or a indolinophthalide compound, and a color developer such as a phosphoric acid compound, an aliphatic carboxylic compound or a phenolic compound. The entire disclosures of both of these references are hereby incorporated by reference.
U.S. Pat. No. 5,480,482 (to Novinson), incorporated by reference herein, discloses compounds of the second type, that is, which are alternately heated or cooled to effect erasure or writing. In particular, the '482 patent discloses a pigment mixture including (a) a colorless cyclic aryl lactone dye that undergoes ring opening to form a colored triarylmethylene carboxylic acid dye, (b) an alkaline activator agent that effects ring opening of the dye when the mixture is heated and ring closure to the colorless lactone state when the mixture is cooled, and (c) a low-melting solid that functions as a solvent and activator. Generally, the dye is rendered colorless through heating to temperatures of 30-70.degree. C., and colored through cooling to temperatures below 25.degree. C. Thus, the mechanism is analogous to that described in the '460 and '669 patents, but the different transition temperatures result in functionally different behavior.
One mechanism for systematically modulating the transition temperature within a range of possibilities involves selective formulation of the low-melting solvent. In solid form, the solvent immobilizes the various reactants, preventing or impeding reaction and consequent transition of the dye between colored and transparent states. Thus, if the solvent melting point is higher than the normal transition temperature of the dye, the transition temperature of a pigment made by combining the dye and the solvent into a particle will be the melting point of the solvent. Hysteresis can be achieved, for example, if the product of the forward reaction (e.g., a ring-opened lactone dye) depresses the freezing point of the solvent to a greater degree than does the product of the reverse reaction (e.g., the closed-ring lactone dye). In this case, following melting of the solvent and occurrence of the forward reaction, the composition must be cooled to a temperature below the previous melting point to achieve solvent solidification. So long as the reverse reaction takes place above this depressed solidification temperature, it will occur prior to renewed reactant immobilization.
In U.S. Pat. No. 4,717,710, the entire disclosure of which is hereby incorporated by reference, a thermochromic composition comprising an electron-donating chromogenic material, a 1,2,3-triazole compound and a weakly basic, sparingly soluble azomethine or carboxylic acid primary amine salt are combined with a solvent such as stearic acid glyceride, lanolin, diphenylphthalate, lauric acid glyceride, propyl laurate or palmitic acid glyceride. Depending on the solvent chosen, the transition temperature can occur over a wide range from about -40.degree. C. to about 150.degree. C.
To the extent these reversible compounds have been used in computer printing applications, it has been in the traditional manner without multiple colors and without gray scale. Most computer-driven printing devices, such as laser, dot-matrix and ink-jet printers, print in a binary fashion: the output medium is divided into an array of picture elements, or "pixels," and the devices can either print a small colored dot at each pixel location or leave the location blank. In the case of monochrome printers, all of the dots are printed in a single color, whereas color printers select a dot color from a small set of colors.
Pictorial imagery, by contrast, is continuous in tonality, exhibiting a "gray scale" that extends over a large range of tonal levels. If such a "continuous-tone" image is divided into pixels, each will exhibit a gray-scale tone. In order to reproduce the image by means of an electronic printing device capable of applying only a single-toned dot, these gray-scale tones must be converted into a binary format. The conversion process, which may take many forms, is generically referred to as "halftoning." A halftone image consists solely of a spatial arrangement of binary colored or uncolored dots, but the human visual system integrates this pattern to create an illusion of a continuous-tone original. Because information is always lost, however, a halftoned image inevitably exhibits visual distortions due to "quantization error" (i.e., the difference between the input tonal value represented by a multibit word and the output value represented by a single bit) wherever the original image contains gray values; in an 8-bit gray scale, for example, such error is absent only at signal levels of 0 or 255.